1. Field of the Invention
The present invention relates to a multilevel NAND type flash memory and a write method of the same.
2. Description of the Related Art
A NAND type flash memory is useful as a large-capacity nonvolatile memory (e.g., Jpn. Pat. Appln. KOKAI Publication No. 2005-235260). Recently, a multilevel NAND type flash memory that stores data having a plurality of bits in one memory cell has been proposed.
Unfortunately, threshold distributions held in memory cells of this multilevel NAND type flash memory have the problem that the width of a threshold distribution corresponding to each individual data and the interval between the threshold distributions of different data are small, and this makes it difficult to ensure the data reliability. In particular, micropatterning of memory cells makes it most difficult to assure the reliability of memory cells connected to a word line WL0 adjacent to a selection gate line on the source line side.
Micropatterning also increases the influence of coupling between charge storage layers of adjacent memory cells; the threshold value of a memory cell in which data is already written is influenced by the fluctuation in threshold value of an adjacent memory cell in which data is written later.
In the multilevel NAND type flash memory that holds data having three or more bits in one memory cell, the reliability of memory cells adjacent to the selection gate line and the coupling effect between adjacent memory cells are serious problems.